The metal lines over insulators and ground planes, or metal lines buried in close proximity to dielectric insulators and used for integrated circuit interconnects are in reality transmission lines or strip lines. The use of coaxial interconnection lines for interconnections through the substrate in CMOS integrated circuits can also be termed transmission lines or strip lines. Interconnection lines on interposers or printed circuit boards can also be described as transmission lines.
The low characteristic impedance of any of these lines, transmission, strip lines or coaxial lines results in part from the low characteristic impedance of free space, Zo=(μo/εo)1/2=377 ohms, and in part from the dielectric material used for electrical insulation in the lines which has a higher dielectric permittivity than free space. Most commonly used coaxial lines have an impedance of 50 ohms or 75 ohms, it is difficult to achieve larger values. In the past these effects have not received much consideration on the integrated circuits themselves since the propagation speed with oxide insulators is 15 cm/ns and switching speeds on integrated circuits of the size of a centimeter have been slower than 1/15 ns or 70 picoseconds. Transmission line effects only become important if the switching time is of the same order as the signal propagation time. Switching times in CMOS circuits have been limited by the ability to switch the capacitive loads of long lines and buffers, and charge these capacitances over large voltage swings to yield a voltage step signal.
Most current CMOS integrated circuit interconnections rely on the transmission of a voltage step or signal from one location to another. FIG. 1 illustrates R-C limited, short high impedance interconnections with capacitive loads.
The driver may simply be a CMOS inverter as shown in FIG. 1 and the receiver a simple CMOS amplifier, differential amplifier, or comparator.
As shown in FIG. 1, the CMOS receiver presents a high impedance termination or load to the interconnection line. This is problematic in that:
(i) the switching time response or signal delay is determined mainly by the ability of the driver to charge up the capacitance of the line and the load capacitance,
(ii) the line is not terminated by its characteristic impedance resulting in reflections and ringing,
(iii) large noise voltages may be induced on the signal transmission line due to capacitive coupling and large voltage swing switching on adjacent lines, the noise voltage can be a large fraction of the signal voltage.
The transmission of voltage step signals only works well if the interconnection line is short so that the stray capacitance of the line is small. Long lines result is slow switching speeds and excessive noise due to capacitive coupling between lines.
FIG. 1 shows the commonly used signal interconnection in CMOS integrated circuits, where voltage signals are transmitted from one location to another. This is problematic in that the interconnection lines are normally loaded with the capacitive input of the next CMOS stage and the large stray capacitance of the line itself. The response time is normally slow due to the limited ability of the line drivers to supply the large currents needed to charge these capacitances over large voltage swings. These times are usually much larger than the signal transmission time down the line so a lumped circuit model can be used to find the signal delay, as shown in FIG. 1.
In the example here the output impedance of the source follower is 1/gm=1000 ohms, and a line 0.1 cm long will have a capacitance of about 0.2 pF if the dimensions of the line are about 1 micron by 1 micron and the insulator or oxide thickness under the line is 1 micron. This results in a time constant of 200 pS and it takes about 400 pS to charge the line from 10% to 90% of the final voltage value. This is a relatively slow response.
Furthermore, if two interconnection wires are in close proximity then the voltage swing on one line can induce a large voltage swing or noise voltage on the adjacent line as shown in FIG. 1. The noise voltage is just determined by the capacitance ratios, or ratio of interwire capacitance, Cint, to the capacitance of the interconnection wire, C.
In prior art these can be comparable, as shown, and depend on the insulator thickness under the wires and the spacing between the wires. Therefore, the noise voltage can be a large fraction of the signal voltage if the wires are in close proximity and far removed from the substrate by being over thick insulators. The emphasis in prior art has always been in trying to minimize the capacitance of the interconnection line, C, by using thick insulators and low dielectric constant materials.
Thus, there is a need to provide a solution for these types of problems for CMOS-scaled integrated circuits. Due to the continued reduction in scaling and increases in frequency for transmission lines in integrated circuits such solutions remain a difficult hurdle. For these and other reasons there is a need to reduce noise in high speed interconnections.